Hybrid powertrain speed control

ABSTRACT

A vehicle powertrain includes a controller, a torque converter, and an engine and electric machine coupled by a clutch. The torque converter may be configured to couple the electric machine to an output shaft. The controller may be programed to generate a command for the electric machine to output torque to drive the torque converter toward a desired rotational speed, and to modify the command according to a difference between the desired rotational speed and an actual rotational speed to reduce the difference, wherein values of the difference are limited by thresholds that change with powertrain operation.

TECHNICAL FIELD

This disclosure relates to hybrid powertrain control systems.

BACKGROUND

Vehicles having automatic transmissions change gear ratios based onvehicle speed and driver acceleration demand. During decelerationautomatic transmissions shift through progressively lower gear ratios.The smoothness of the shifting from a given gear to the next adjacentgear influences noise, vibration, and harshness as perceived by thedriver.

Hybrid vehicles may employ one or more electric machines having amotor-generator in combination with an internal combustion engine.Depending on vehicle operating conditions, the electric machine mayselectively alternate between serving as a motive power source, or adecelerating load upon the powertrain.

SUMMARY

A vehicle powertrain includes a controller, a torque converter, and anengine and electric machine coupled by a clutch. The torque convertermay be configured to couple the electric machine to an output shaft. Thecontroller may be programed to generate a command for the electricmachine to output torque to drive the torque converter toward a desiredrotational speed, and to modify the command according to a differencebetween the desired rotational speed and an actual rotational speed toreduce the difference, wherein values of the difference are limited bythresholds that change with powertrain operation.

A hybrid powertrain control method includes, by a controller, generatinga command and modifying the command. The controller generates thecommand for an electric machine to output a desired rotational speedthat is based on a torque estimate. And the controller modifies thecommand according to a difference between the desired rotational speedand an actual rotational speed to reduce the difference, wherein valuesof the difference are limited to a range that is based on hybridpowertrain operational mode.

A control system for a vehicle powertrain includes a controllerprogramed to adjust a command for an electric machine according to adifference between a desired rotational speed and an actual rotationalspeed of a torque converter coupled with the electric machine to reducethe difference, wherein values of the difference are limited bythresholds that change with operational mode of the vehicle powertrain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a hybrid electric vehicle powertrain andcontrol system.

FIG. 2 is a block diagram of a powertrain speed control algorithm.

FIGS. 3A and 3B are flowcharts of a powertrain control method.

FIG. 4 is a block diagram of a powertrain speed control algorithm.

FIGS. 5A and 5B are flowcharts of a powertrain control method includinga torque converter model.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

Speed control is typically used in powertrain control systems toregulate the electric machine's or actuator's speed. A control systemcompares a difference between an actuator's speed target and measured(or estimated) speed. The control system then calculates the desiredtorque command to regulate the speed. The control system typically usesa PID strategy, where the P stands for Proportional, I for Integration,D for Derivative. A PID controller has PID gains tuned to a specificoperating environment, which impacts the overall PID performance is alloperating environments.

In a conventional vehicle with an automatic transmission, an engine maybe placed in a speed control mode. In a hybrid electric vehicle, anelectric motor may also be used for speed regulation. The electric motoris a considered a fast regulator to regulate the speed given it canincrease and decrease the torque instantly. A torque command of thecontrol system typically is saturated at estimated torque actuator(s)torque limits. However, saturation of a torque command at the actuator'sphysical limit may not be sufficient to provide smooth operation at somefaulted conditions. For example, a powertrain control system maytypically switch back and forth between speed and torque controldepending on driver maneuver and a powertrain system state. In othercases, one actuator may be always in speed control. A multitude of noisefactors may also cause undesired vehicle response with the standardspeed control. For example, an error mode command that is when thecontrol system should be in torque control mode, however, the systemoperates in speed control. Here, the control system operates to follow agiven speed target and if the speed target is not at the actual speed,the torque command from the speed controller may increase to a levelthat causes a significant and noticeable disturbance to the vehicle.Another example is a speed target command error, that is when the systemcommands an incorrect speed target, which may be far from the actualspeed that possibly causes a significant and noticeable disturbance tothe vehicle. A further example is an actual speed measurement error inwhich, during the transition from torque control to speed control, ameasurement of speed has error, the speed target may be also far fromthe actual speed, which causes undesired vehicle response.

Here, methods for speed and torque control are presented to reduce andmitigate potential undesired vehicle responses during error conditions.Those methods include limiting target speed to a range of measured speedsuch that ω_(actl)−Δω₁<ω_(target)<ω_(actl)+Δω₂ in which ω_(actl) is themeasured actuator's speed, and Δω₁ and Δω₂ are the differences in speedof target speed from the actual speed. Δω₁ and Δω₂ may depend onpowertrain/vehicle operation conditions. Another method includeslimiting torque command of the speed controller to a specific range suchthat Trq_(SpdCntl)=Trq_(FdFrd)+Trq_(Fdbck) and the torque may be limitedto Trq_(min)<Trq_(SpdCntl)<Trq_(Max) Or the feedback torque may belimited to Trq_(FbMin)<Trq_(Fdbck)<Trq_(FbMax)

Here, a PID controller gain may be adjusted to reflect saturation oftorque on these limits. These limits may be subject to the normalsystems torque limits that may be calibratable depending on powertrainconfiguration or vehicle operation modes. These limit may be selectedsuch that they are large enough to reject noise when in normal speedcontrol and small enough to reduce unacceptable vehicle responses duringerror conditions.

FIG. 1 schematically depicts a hybrid electric vehicle (HEV) 10, andillustrates representative relationships among vehicle components.Physical placement and orientation of the components within the vehiclemay vary. The vehicle 10 includes a powertrain 12 having an engine 14that drives a transmission 16. As will be described in further detailbelow, the transmission 16 includes an electric machine such as anelectric motor/generator (M/G) 18, a torque converter 22, and a multiplestep-ratio automatic transmission, or gearbox 24. There is also a highvoltage traction battery 20 in connection with the M/G 18 for providingpower to, and receiving power from, the M/G 18.

The engine 14 and the M/G 18 are both capable of providing motive powerfor the HEV 10. The engine 14 generally represents a power source whichmay include an internal combustion engine such as a gasoline, diesel, ornatural gas powered engine, or a fuel cell. The engine 14 generatespower and a corresponding output torque that is supplied to the M/G 18when a disconnect clutch 26 between the engine 14 and the M/G 18 is atleast partially engaged. The M/G 18 may be implemented by any one of aplurality of types of electric machines. For example, M/G 18 may be apermanent magnet synchronous motor. Power electronics 28 conditiondirect current (DC) power provided by the battery 20 to the requirementsof the M/G 18, as will be described below. For example, powerelectronics may provide three phase alternating current (AC) to the M/G18. Additionally, there is a DC/DC converter 56 which steps down voltagefrom the high voltage battery 20 for powering other smaller vehicleloads. In at least one embodiment the DC/DC converter conditions powerto supply an auxiliary transmission pump and a low voltage enginestarter motor.

Transmission 16 is operable to deliver a variable gear ratio. Thegearbox 24 may include internal gear sets (not shown) that are placed indifferent gear ratios by selective engagement of friction elements suchas clutches and brakes (not shown) to establish the desired multiplediscrete or step drive ratios. The friction elements are controllablethrough a shift schedule that connects and disconnects certain elementsof the gear sets to control the ratio between a gearbox input shaft 34and the transmission output shaft 38. The gearbox 24 ultimately providesthe powertrain output torque through the output shaft 38. For example,there may be two series of clutches, where each corresponds toodd-numbered or even-numbered gear sets. During shifting from a currentgear ratio to an adjacent requested gear ratio, a clutch from the firstseries is concurrently disengaged while a clutch from the second seriesis engaged. Once the transfer from the first to the second clutch iscompleted as part of a gear shift, both the speed ratio and torque ratiobetween the transmission output shaft 38 and the transmission inputshaft 34 changes according to the gear selection.

As further shown in the representative embodiment of FIG. 1, the outputshaft 38 is connected to a differential 40. The differential 40 drives apair of wheels 42 via respective axles 44 connected to the differential40. The differential transmits torque allocated to each wheel 42 whilepermitting slight speed differences such as when the vehicle turns acorner. Different types of differentials or similar devices may be usedto distribute torque from the powertrain to one or more wheels. In someapplications, torque distribution may vary depending on the particularoperating mode or condition, for example.

The vehicle 10 further includes a foundation brake system 54. The brakesystem may comprise friction brakes suitable to selectively applypressure by way of stationary pads attached to a rotor affixed to eachwheel. The applied pressure between the pads and rotors creates frictionto resist rotation of the vehicle wheels 42, and is thereby capable ofslowing the speed of vehicle 10.

When the disconnect clutch 26 is at least partially engaged, power flowfrom the engine 14 to the M/G 18 or from the M/G 18 to the engine 14 ispossible. For example, when the disconnect clutch 26 is engaged, the M/G18 may operate as a generator to convert rotational energy provided by acrankshaft 30 through M/G shaft 32 into electrical energy to be storedin the battery 20. As discussed in more detail below, the rotationalresistance imparted on the shaft through regeneration of energy may beused as a brake to decelerate the vehicle. The disconnect clutch 26 canalso be disengaged to decouple the engine 14 from the remainder of thepowertrain 12 such that the M/G 18 can operate as the sole drive sourcefor the vehicle 10.

A main transmission pump 46 is used to operate the transmission 16 whenthe engine 14 is operating. Additionally, the main transmission pump 46is powered by the vehicle engine 14. The main pump 46 draws fluid from asump in the bottom of the transmission 16 and generates pressure withinthe hydraulic system. The main transmission pump 46 is generally poweredby the M/G 18. Pressure is provided to the transmission and thedisconnect clutch as long as the M/G is spinning at a high enough speed.When the M/G is stopped, an auxiliary electric pump 52 provides certainfunctions of the main transmission pump 46 and supports some limitedtransmission operation. Therefore, the disconnect clutch 26, as well asother transmission mechanisms, can be engaged by the auxiliary pump 52to maintain functionality during certain operating conditions.

Operation states (also referred to as operational modes) of thepowertrain 12 may be dictated by at least one controller. In at leastone embodiment, there is a larger control system including severalcontrollers. The individual controllers, or the control system, may beinfluenced by various other controllers throughout the vehicle 10, wherea vehicle system controller (VSC) 48 operates at a higher hierarchyrelative to other subservient controllers. The VSC 48 output maydirectly or indirectly dictate or influence a number of vehiclefunctions such as starting/stopping engine 14, operating the M/G 18 toprovide wheel torque or recharge the traction battery 20, select orschedule transmission gear shifts, etc. For example, the VSC 48 mayreceive data from, and issue commands to, other subservient controllersthat may operate lower in a controller hierarchy relative to the VSC 48.For example, other controllers which communicate with the VSC include atransmission control module (TCM), brake system control module (BSCM), ahigh voltage battery energy control module (BECM), an inverter systemcontroller (ISC), as well as other controllers in communication whichare responsible for various vehicle functions. In at least oneembodiment, the BECM and the ISC are included within power electronics28.

Any of the above-mentioned controllers may further include amicroprocessor or central processing unit (CPU) in communication withvarious types of computer readable storage devices or media. Computerreadable storage devices or media may include volatile and nonvolatilestorage in read-only memory (ROM), random-access memory (RAM), andkeep-alive memory (KAM), for example. KAM may be implemented aspersistent or non-volatile memory that may be used to store variousoperating variables while the CPU is powered down, or it may beimplemented as battery backup powered RAM. Computer-readable storagedevices or media may be implemented using any of a number of knownmemory devices such as PROMs (programmable read-only memory), EPROMs(electrically PROM), EEPROMs (electrically erasable PROM), flash memory(e.g., NAND FLASH or NOR FLASH), or any other electric, magnetic,optical, or combination memory devices capable of storing data, some ofwhich represent executable instructions, used by the controller incontrolling the engine or vehicle.

The VSC 48 and other controllers communicate with various engine/vehiclesensors and actuators via an input/output (I/O) interface that may beimplemented as a single integrated interface that provides various rawdata or signal conditioning, processing, and/or conversion,short-circuit protection, and the like. Alternatively, one or morededicated hardware or firmware chips may be used to condition andprocess particular signals before being supplied to the CPU. Asgenerally illustrated in the representative embodiment of FIG. 1, theVSC 48 may communicate signals to and/or from electronics within thehigh voltage battery 20 and the power electronics 28. The powerelectronics 28 may comprise both the ISC and the BECM that manage powerflow to and from the battery 20. Additionally, the VSC 48 maycommunicate with other vehicle controllers as discussed above, ordirectly with vehicle sensor and/or components including the engine 14,the braking system 54, the DC/DC converter 56, the low voltage battery58, and starter 60. Although not explicitly illustrated, those ofordinary skill in the art will recognize various functions or componentsthat may be controlled by the VSC 48 within each of the subsystemsidentified above.

Representative examples of parameters, systems, and/or components thatmay be directly or indirectly actuated using control logic executed bythe controller include fuel injection timing, rate, and duration,throttle valve position, spark plug ignition timing (for spark-ignitionengines), intake/exhaust valve timing and duration, front-end accessorydrive (FEAD) components such as an alternator, air conditioningcompressor, battery charging, regenerative braking, M/G operation,clutch pressures for disconnect clutch 26, torque converter bypassclutch 36, and transmission gearbox 24, and the like. Sensorscommunicating input through an I/O interface may be used to indicatecrankshaft position, engine rotational speed (RPM), M/G shaft speed,powertrain output shaft speed, wheel speeds, engine coolant temperature,intake manifold pressure, accelerator pedal position, ignition switchposition, throttle valve position, air temperature, exhaust gas oxygenor other exhaust gas component concentration or presence, intake airflow, transmission gear, ratio, or mode, transmission oil temperature,transmission turbine speed, torque converter bypass clutch status,deceleration, or shift mode, for example.

The VSC 48 also includes a torque control logic feature. The VSC 48 iscapable of interpreting driver requests based on several vehicle inputs.These inputs may include, for example, gear selection (PRNDL),accelerator pedal inputs, brake pedal input, battery temperature,voltage, current, and battery state of charge (SOC). The VSC 48 in turnmay issue command signals to the power electronics 28 to influence theoperation of the M/G 18.

The M/G 18 is also in connection with the torque converter 22 via shaft32. Therefore, the torque converter 22 is also connected to the engine14 when the disconnect clutch 26 is at least partially engaged. Thetorque converter 22 includes an impeller fixed to the M/G shaft 32 and aturbine fixed to a transmission input shaft 34. The torque converterfluidly couples the electric machine to the output shaft. Morespecifically, the torque converter 22 provides a hydraulic couplingbetween shaft 32 and transmission input shaft 34. An internal bypassclutch 36 may also be provided torque converter such that, when engaged,clutch 36 frictionally or mechanically couples the impeller and theturbine of the torque converter 22, permitting more efficient powertransfer. The torque converter 22 and its bypass clutch 36 may bereplaced with a launch clutch to provide vehicle launch. In contrast,when the bypass clutch 36 is disengaged, the M/G 18 may be mechanicallydecoupled from the differential 40 and the vehicle axles 44. Forexample, during deceleration the bypass clutch 36 may disengage at lowvehicle speeds, decoupling the engine from the transmission anddriveline, to allow the engine to idle and operate at low vehicle speedsor stop. The timing and degree of the alternate operation of the M/G mayserve to optimize fuel economy, and should coincide with the gearshifting operations of the transmission.

A driver of the vehicle 10 may provide input at accelerator pedal 50 andcreate a demanded torque, power, or drive command to propel the vehicle10. In general, depressing and releasing the pedal 50 generates anaccelerator input signal that may be interpreted by the VSC 48 as ademand for increased power or decreased power, respectively. Based atleast upon input from the pedal, the controller 48 may allocate torquecommands between each of the engine 14 and/or the M/G 18 to satisfy thevehicle torque output demanded by the driver. The controller 48 may alsocontrol the timing of gear shifts within the gearbox 24, as well asengagement or disengagement of the disconnect clutch 26 and the torqueconverter bypass clutch 36. The torque converter bypass clutch 36 can bemodulated across a range between the engaged and disengaged positions.This may produce a variable slip in the torque converter 22 in additionto the variable slip produced by the hydrodynamic coupling between theimpeller and the turbine. Alternatively, the torque converter bypassclutch 36 may be operated as either locked or open without using amodulated operating mode depending on the particular application.

The driver of vehicle 10 may additionally provide input at a brake pedalto create a vehicle braking demand. Depressing the brake pedal generatesa braking input signal that is interpreted by controller 48 as a commandto decelerate the vehicle. The VSC 48 may in turn issue commands tocause the application of negative torque to the powertrain output shaft38. Additionally or in combination, the controller may issue commands toactivate the brake system 54 to apply friction brake resistance toinhibit rotation of the vehicle wheels 42. The negative torque valuesprovided by both of the powertrain and the friction brakes may beallocated to vary the amount by which each satisfies driver brakingdemand.

The M/G 18 may operate as a generator to convert kinetic energy from thepowertrain 12 into electric energy to be stored in the battery 20. TheM/G 18 may act as a generator while the engine 14 is providing the solepropulsion power for the vehicle 10, for example. The M/G 18 mayadditionally act as a generator during times of regenerative braking inwhich rotational energy from the spinning of the output shaft 38 istransferred back through the gearbox 24, and is converted intoelectrical energy for storage in the high voltage battery 20 or the lowvoltage battery 58.

It should be understood that the schematic illustrated in FIG. 1 ismerely exemplary and is not intended to be limiting. Otherconfigurations are contemplated that utilize selective engagement ofboth an engine and a motor to transmit through a transmission. Othersuch configurations are contemplated without deviating from the scope ofthe present disclosure.

Generally, the electric machine has a higher torque bandwidth comparedto the engine. Therefore, the engine generally operates in torquecontrol, and the electric machine may be operated using speed control tomore precisely control overall powertrain output speed when needed.According to a powertrain of the present disclosure, the engine dictatesthe system power output and the electric machine regulates system speed.In this case, the electric machine is used to modulate overall speedoutput of the powertrain system.

Effectiveness of using the electric machine to modulate system speed canbe monitored by measuring the torque and speed at impeller input of thetorque converter 22. Generally, the torque of torque controller impellermay be described by equation (1) below.

τ_(imp)=τ_(e)+τ_(m)  (1)

In equation (1), τ_(imp) is the impeller torque, τ_(e) is the engineoutput torque, and τ_(m) is the output torque of the electric machine.τ_(m) is positive if the electric machine assists the engine in vehiclepropulsion. τ_(m) is negative when the electric machine operates as agenerator and absorbs a portion of the engine output torque.

The dynamics that govern the output of the electric machine can beapproximated by equation (2) below.

J _(m)·{dot over (ω)}_(m)=τ_(clt)+τ_(m)+τ_(tc)  (2)

where J_(m) is the electric machine inertia, {dot over (ω)}_(m) isrotational acceleration output from the electric machine, τ_(clt) is theclutch torque, and τ_(tc) is the torque load from the torque converter.The clutch torque τ_(clt) will be the amount of torque applied to theelectric machine, and depends on whether the clutch is open, closed, orin a slip mode.

In the simplest case, when the clutch is open, there is zero torquetransferred across the clutch. If the clutch is locked, the clutchtorque capacity is high enough so that the torque transferred throughthe clutch is equal to the engine brake torque minus engine inertia.Equation (3) below is a representative estimate of clutch torque in alocked state.

τ_(clt)=τ_(e) −J _(e)·{dot over (ω)}_(e)  (3)

If the clutch is in a slip mode the clutch torque equals the torquecapacity, and is a function of the surface condition of the clutchplates), and the hydraulic pressure p applied across the clutch.Equation (4) below represents a typical relationship.

τ_(clt) =f(λ,p)  (4)

As discussed above, engine torque output can overshoot and/or undershootan ideal value as the state of the clutch changes. The powertrain systeminertia varies significantly between electric only mode and hybridvehicle modes. It is beneficial to use both feedforward information fromthe disconnect clutch as well as feedback of any electric machine speederror to manage powertrain speed output. According to aspects of thepresent disclosure, motor control may use a preliminary estimate ofτ_(clt) as feedforward information to counteract changes in clutchtorque related to state changes. Also, feedback from the actual outputspeed may be used to reject disturbances and limit error in speedoutput. A speed domain transfer function is represented below inequation (5).

τ_(m)(s)=τ_(clt) ^(est)(s)+G _(m)(s)·(ω_(m) ^(r)(s)−ω_(m)(s))  (5)

τ_(clt) ^(est)(s) is the estimated torque across the clutch, which iscontinuously adjusted by feedback terms based on the actual powertrainoutput. ω_(m) ^(r)(s) is the commanded rotational speed of the electricmachine, and ω_(m)(s) is the measured actual rotational speed output ofthe electric machine. The difference between the commanded electricmachine speed and the actual electric machine speed is input into anadjustment portion of the algorithm, G_(m)(s). This adjustment allowsthe system to overcome clutch torque estimation errors.

The torque estimation algorithm is represented in the control systemdiagram of FIG. 2. System 200 represents information flow betweencontrollers within a system that employs a combination of bothfeedforward and feedback information to control powertrain system speed.

A feedforward portion of the algorithm is represented at clutch control202. Based on information from the engine 204 (i.e., output torque andspeed) and information from the clutch 206 (i.e., lock state, surfacecondition, and hydraulic pressure), an estimate clutch torque 208 isprovided. The motor control 210 uses the clutch torque estimate togenerate a command to regulate rotational speed of the electric machine.In practice the clutch torque estimate 208 is often erroneous due tochanges in system dynamics.

Specifically, motor dynamics 212 heavily affects the actual electricmachine speed output 214. Actual electric machine output torque 216 andactual clutch torque 218 are affected by the electric machine inertia220 and vehicle speed and acceleration profiles 222. The actual electricmachine speed output 214 is measured and fed back to the motor speedcontrol 210. Any error between the commanded rotational speed 224 of theelectric machine and the actual speed output 214 of the electric machineis used to modify the motor torque command. The measured error,including any electric machine noise 226, is fed back to the motor speedcontrol 210 to operate as an error correction. The magnitude of thiscorrection depends on the control system gain 228. This then combineswith the clutch torque estimate 208 to generate a motor torque commandwhich minimizes the error between the speed command and the actualspeed. Overall the feedback control operates to improve powertrainsystem robustness and transient response.

Generally, the feedback portion of the control algorithm comprises errorcorrection to compensate for, and reduce speed discrepancies, improvingrobustness. However, the feedback portion is reactive, and there is someinherent time delay in the correction. The feedforward portion of thecontrol algorithm predicts performance and avoids some portion of theoutput speed error. As inputs change, the downstream speed output ispredicted before it arises. Feedforward control can compensate to somedegree for known motor dynamic before the errors occur, reducing systemdelays. However, feedforward control is most effective when systemresponse is highly predictable. The advance estimation of clutch torquecan carry error for several reasons. For example, the loss of a signalindicative of clutch pressure, loss of communication between controllerswithin the controller system, poor general signal conditions, and longcommunication delay all can contribute to errors in the clutch torqueestimate. When both control types are used together, the feedforwardcomponent helps to provide rapid system response, and the feedbackcomponent helps to compensate for unavoidable errors in the powertrainsystem model. The method of the present controller provides a robust wayto control hybrid powertrain output.

Speed control mode may be applied in the powertrain of the presentdisclosure to improve operation in several specific operation states.FIGS. 3A and 3B are a flowchart of method 300 which illustrates anexample of mode determination of speed control in the various powertrainoperation states.

At step 302, if the powertrain is not in a formal speed control mode,electric machine speed control may still be used to protect againstcertain potential powertrain fault conditions to protect the engineand/or the transmission.

At step 304, if the electric machine speed is less than a firstthreshold, such as when vehicle is coming to a stop, the controller maycommand the electric machine to be stopped to save energy. At step 306,if the electric machine torque is negative, the controller may commandthe electric machine to smoothly ramp down to zero speed at 308. In atleast one embodiment, the controller, in response to an imminent vehiclestop, causes a prompt ramp down by generating a smooth reduction of thespeed target command for the electric machine. The controller may usethe electric machine torque to regulate the speed until it is closeenough to zero speed. A dead band of the speed control is implemented atvery low speeds to fully cut the electric machine torque down to zeroand let the electric machine speed reduce to stationary by itself withthe help from the resistance of cooling oil.

At step 306, if the electric machine torque is positive, the dead bandmay be single-sided at step 310, also having a zero speed target. Theadvantage of dead band control at low speeds is that it ensures that theelectric machine torque is zero when the electric machine is fullystopped. For example, electric machine rotational speeds less than about20 rpm may be subject to the dead band to allow the speed to remain atzero when desired. At such low speeds, tighter control may be requiredbecause noise in speed measurement has a greater impact on systemcorrection. The use of a dead band avoids small torque oscillation atzero speed due to measurement or calculation having accumulative errors.When otherwise using error correction at very low speeds, the errors onthe electric machine speed can lead to ongoing small adjustments oftorque, which are both unnecessary and inefficient.

At step 304, if the motor speed is greater than the first threshold, thecontroller may still protect for a minimum oil pressure in thetransmission. For example, when the torque converter is closed, theimpeller speed may be regulated to protect high priority operation ofthe transmission. The transmission requires the impeller speed to behigher than a value that can guarantee a sufficient oil pressure.Normally it is between 300-400 rpm. When the system is in torquecontrol, inaccuracy in the torque output can lead to a drop in impellerspeed. If at step 312, the impeller speed is less than a pressurethreshold speed, the controller may invoke transmission speed protectionat step 314 to increase the electric machine output to ensure that theimpeller speed is greater than a predetermined minimum impeller speed.In a second example, a similar speed protection may be applied tocontrol impeller speed when at zero to prevent the impeller speed fromgoing negative in the event of multiple control/system failure.

At step 312, if the impeller speed is greater than the pressurethreshold speed, the controller may still protect for a minimum idlespeed of the engine and electric machine, even when the powertrain is intorque control mode. A normal operation in torque control mode will sendtorque command to both the engine and the motor. However, if thecalculation is incorrect or the actuators are under-delivering orover-delivering, the combined torque could be in wrong direction whichleads to a sharp drop of the speed below the idle, especially when thetorque converter by-pass clutch is open. While the engine is running andthe disconnect clutch is locked, the controller may command motorrotational speed to maintain engine speed above an idle speed threshold.If at step 316, the engine speed is less than an idle threshold speed,the controller may invoke idle speed protection at step 318 to increasethe electric machine output to maintain the engine speed above thethreshold.

According to aspects of the methods described herein, the controller mayprotect the powertrain speed from falling below a desired idle speedwhen the engine is on, from falling under the minimum speed fortransmission pressure in EV mode, and from falling below zero in all usecases.

If at step 302, speed control is enabled, the controller may command theelectric machine output speed to reduce powertrain harshness undervarious operating conditions. If at step 320, the vehicle powertrain isat idle, the controller may invoke idle speed control for both EV modeand HEV mode.

If at step 322, there is no imminent engine start, the feedforward termof the idle speed control may be zero when the engine is off. Therefore,the electric machine torque is used to directly regulate the impellertorque at step 324, for example, by using the feedback calculation inequation (5) discussed above where the estimated clutch torque is zero.

While the powertrain is operated in EV mode, several conditions mayinvoke an engine start to supplement power. For example, when thevehicle is operated in EV mode at idle, a low SOC may require the engineto be started to avoid the SOC decreasing to less than a predeterminedcharge threshold. Similarly, the engine may be required to be startedwhen the powertrain is at idle in EV mode is to provide power for highelectrical loads, such as an air conditioner compressor. In a furtherexample, a steep increase in driver acceleration demand, or acceleratorpedal tip-in, may exceed the capabilities of the electric machine outputtorque. An engine start may be required to supplement propulsion torque.However, when the engine is started it may cause a significant load onthe motor and disrupt system speed. If at step 322, there is an imminentengine start, the motor speed may be ramped up to prepare fordisturbance rejection. For example, the disconnect clutch torquecalculated by equation (4) may be used as the feedforward term. In atleast one embodiment, the controller may command a ramp up of therotational speed of the electric machine at step 326 in response to animminent engine start event. The ramp up may be characterized by a speedprofile having a rate of change that is less than a predeterminedroughness threshold. This speed ramp up may operate to minimizediscrepancies in powertrain output torque during the engine start event.

If the vehicle is in speed control mode at low speeds above idle at step320, there may be a targeted electric machine rotational speed outputused at step 328 in response to an imminent gear ratio shift of thetransmission. The target speed control, which is referred to as“Target-N”, is a special mode of the speed controller when thetransmission initiates a downshift before the torque converter is fullyopen. The controller is programmed to maintain the impeller speed at apredetermined rotational speed, or Target-N, step 330 to deliver asmooth torque transition during a gear shift event. In such case, thespeed control gains will not be the same as those used in idle speedcontrol discussed above. Target-N needs a set of much less aggressivegains to achieve the target of holding the impeller speed to minimizetorque impact to the driveline. In at least one embodiment, Target-N isa fixed value used during the shift event to dictate motor speed tostabilize impeller speed. One example application is during atransmission downshift at very low vehicle speeds greater than idle. Inanother example, Target-N may include a speed profile that varies duringthe shift event to compensate for torque surges during the shift event.Similar to other aspects of the present disclosure, the speed profilemay include a rate of change less than a roughness threshold to promotesmooth transitions.

If at step 328, Target-N control is not being used, the controller mayemploy special conditions if the speed control is resuming from zero. Ifat step 332, the speed control is resuming, both speed resumptionprofile shaping may be used, as well as single-side dead band control.Both aspects are discussed in more detail below.

At step 334, the controller may command the initial speed to ramp up toaccording to a predetermined profiled to mitigate potential clunking dueto backlash in the gear system, as well as shuffle due to drivelineresonance. A control mechanism called input shaping can be applied tothe motor target speed. The speed ramp up rate needs to be designed tobe able to change with respect to the actual properties of the drivelinesystem. Calibration or control parameter tuning can be used to adjustboth of the speed ramp up rate and filtering so that a smooth torqueresponse can be achieved.

Applying a speed profile to the electric machine output may also be usedto coordinate with the transmission engagement when the gear lever isfirst shifted into a motive gear such as “Drive” or “Reverse,” from anon-motive gear such as “Park” or “Neutral.” The transmission oilpressure is generally proportional to the torque converter impellerspeed. A desirable speed profile may lead to rapid resumption of the oilpressure while still allowing the transmission control to havesufficient time to engage all the clutches. Before the transmissionengagement is finished, the impeller speed should not be raised again.Otherwise, a clunking feel may be observed by a vehicle operator.

At step 334, the controller may additionally invoke a single-speed deadband control in response to the speed control resuming from zero. At lowimpeller speeds, the overall control architecture as shown in FIG. 2 maybe changed to disable the feedforward term. The feedforward clutchtorque estimate is not accurate at speeds less than the engine idlespeed, therefore the reported engine torque may also be inaccurate. Atimpeller speeds less than the minimum engine combustion speed, enginestart may also be inhibited. In this case, the feedforward clutch torqueestimation term could lead to unexpected conditions such as anundesirable speed dip to negative from zero where speed resumption hasbeen commanded. In order to deliver a smooth ramp up without the dangerof leading to a negative impeller speed when the motor speed is low, thespeed control is designed to be single-sided. In this way, only themotor torque is used to make the motor speed increase from zero to idle,at least to a value where the speed has increased to close enough toidle speed.

At step 332, if the speed control is not resuming, the controller mayinvoke a dead band control as discuss above during low speeds. Duringnormal operation, the feedback portion of a speed controller providesrobustness. However in certain situations, the speed controller feedbacktorque can negatively affect vehicle performance and drivability. Forexample if a torque converter clutch fault causes the clutch to lockwhen open is requested, then it might not be possible for the desiredspeed target to be achieved. The feedback portion of the speedcontroller could grow to an undesired level. To prevent these types ofpotential issues, additional safeguards can be added to the speedcontrol system 200. FIG. 4 is a block diagram of a speed control systemfor a powertrain which include feedback bounding. System 400 representsinformation flow between controllers within a system that employs acombination of both feedforward and feedback information and both torqueand speed bounds to control powertrain system speed.

A feedforward portion of the algorithm is represented at clutch control402. Based on information from the engine 404 (i.e., output torque andspeed) and information from the clutch 406 (i.e., lock state, surfacecondition, and hydraulic pressure), an estimate clutch torque 408 isdetermined (tq_(clc) ^(est)) and provided as negative feedback to amotor control block 410. The motor control 410 uses the clutch torqueestimate to generate a command to regulate rotational speed of theelectric machine. In practice the clutch torque estimate 408 is oftenerroneous due to changes in system dynamics. Along with the clutchcontrol 402, a torque converter model 430 is used to generate anestimated clutch torque (tq_(tcc) ^(est)) that is provided to the motorcontrol block 410.

Another input to the motor control block 410 is a nominal referencemotor speed (ω^(i) _(m)) 432 that has passes through a reference speedbounding block 434. It should be noted that a controller, processor, orother structure that implements this system 400 may view the nominalreference motor speed (ω^(i) _(m)) 432 as a desired rotational speed.When entering speed control, the measured motor speed might besignificantly different from the nominal reference speed (ω^(i) _(m))432. To prevent the system from attempting to apply a large initialspeed control feedback torque, the reference speed can be boundedaccording to equation 6:

ω^(r) _(m)=max(ω_(m) ^(est) −C ₁,min(ω^(i) _(m),ω_(m) ^(est) +C ₂))  (6)

In which (ω^(r) _(m)) is the reference motor speed, (ω_(m) ^(est)) isthe estimated/measured motor speed, (ω^(i) _(m)) is the nominalreference motor speed, C₁ is the predefined minimum reference speeddifference, and C₂ is the predefined maximum reference speed difference.The use of the reference speed bounding 434 ensures that a magnitude ofthe initial speed control error is sufficiently small to produce asmooth transition into speed control. If a bounding correction isrequired, the bounding correction can be described by equation 7:

ω^(r) _(m,corr)=ω^(r*) _(m)−max(ω_(m) ^(est) −C ₁,min(ω^(r*) _(m),ω_(m)^(est) +C ₂))  (7)

In which (ω^(r) _(m,corr)) is the reference motor speed correction. Thuswhile speed control is active, the motor reference speed (ω^(r) _(m))should asymptotically approach the nominal reference speed (ω^(i) _(m)).This can be achieved by decaying the reference motor speed correction tozero using filter such as a first order filter. For example, equations

ω^(r) _(m)=ω^(r*) _(m)−ω^(r) _(m,corr)  (8)

ω^(r) _(m,corr)(t−t ₀)=ω^(r) _(m,corr)(t ₀)e ^(−t/τ) ^(corr)   (9)

In which t is time, t₀ is the time when the speed control was enteredand τ_(corr) is the speed correction time constant.

As, motor dynamics 412 heavily affects the actual electric machine speedoutput 414. And, an actual electric machine output torque 416 and actualclutch torque 418 are affected by the electric machine inertia 420 andvehicle speed and acceleration profiles 422. It should be noted that acontroller, processor, or other structure that implements this system400 may view the actual electric machine output torque 416 as a commandfor the electric machine to output torque or a commanded torque. Theactual electric machine speed output 414 is measured and fed back to themotor speed control 410. Any error between the commanded rotationalspeed 424 of the electric machine and the actual speed output 414 of theelectric machine is used to modify the speed command. The measured erroralso includes any electric machine noise 426, which is fed back to themotor speed control 410 to influence control system gain 428 and operateas error correction. As large speed control errors are not typicallyencountered during normal operation, a speed controller is typicallyoptimized to reject small to moderate speed control errors, while largespeed errors may pose significant more complex challenges. For example,if the proportional gain is too large, then the speed control feedbacktorque can change faster than the system can physically respond,resulting in motor oscillations. Also, large speed error may createintegrator windup when first starting. To prevent these issues, themotor speed error can be bounded by a speed difference limiting block436 having a upper maximum 438 threshold and a lower minimum 440threshold. The motor speed error can be bounded according to equation 10and 11 below

$\begin{matrix}{\frac{d\; \omega_{{err},{ratelim}}}{dt} = {\max \mspace{11mu} \left( {C_{3},{\min \mspace{11mu} \left( {\frac{d\; \omega_{err}}{dt},C_{4}} \right)}} \right)}} & (10) \\{\omega_{{err},{bounded}} = {\max \mspace{11mu} \left( {C_{5},{\min \mspace{11mu} \left( {\omega_{{err},{ratelim}},C_{6}} \right)}} \right)}} & (11)\end{matrix}$

In which (ω_(err)) is the motor speed error, (ω_(err,rate lim)) is therate limited motor speed error, (ω_(err,bounded)) is the bounded motorspeed error, C₃ is the predefined minimum speed error rate of change, C₄is the predefined maximum speed error rate of change, C₅ is thepredefined minimum speed error, C₆ is the predefined maximum speederror. Here, the predefined limits (C₃, C₄, C₅, and C₆) may change as afunction of operating mode. For example, during EV mode, the main sourceof error is the uncertainty in the torque converter model. Because onlyvery small disturbances are expected, the minimum and maximum errorbounds can be narrow. During engine starting when the disconnect clutchtorque estimate is most uncertain, larger error bounds may be assigned.During a coasting shift which uses Target-N control, moderate errorbounds may be assigned.

Next, the bound output is multiplied by a coefficient in the gain stage428 to convert the angular speed error to a torque. After the gain stage428, the torque is bounded according to a post-gain limiter 442. Evenwhen the speed error is bounded in the reference speed bounding 434, andthe speed difference limiting block 436, the feedback control maygenerate large feedback corrections. To prevent these issues, the torquecan be bounded by a torque limiting block 442 having an upper maximum444 threshold and a lower minimum 446 threshold. To ensure the motorresponse is maintained within a bounded, the speed control feedbacktorque can be limited according to equations 12 and 13 below:

$\begin{matrix}{\frac{d\; {tq}_{{fb},{ratelim}}}{dt} = {\max \mspace{11mu} \left( {C_{7},{\min \mspace{11mu} \left( {\frac{d\; {tq}_{fb}}{dt},C_{8}} \right)}} \right)}} & (13) \\{{tq}_{{fb},{bounded}} = {\max \mspace{11mu} \left( {C_{9},{\min \mspace{11mu} \left( {{tq}_{fb},C_{10}} \right)}} \right)}} & (14)\end{matrix}$

In which (tq_(fb)) is the speed control feedback torque,(tq_(fb,rate lim)) is the rate limited speed control feedback torque,(tq_(fb,bounded)) is the bounded speed control feedback torque, C₇ isthe predefined minimum torque rate of change, C₈ is the predefinedmaximum torque rate of change, C₉ is the predefined minimum torque, C₁₀is the predefined maximum torque. The predefined limits can change as afunction of operating mode. For example, during EV mode, the main sourceof error is the uncertainty in the torque converter model. Because onlyvery small disturbances are expected, the minimum and maximum errorbounds can be narrow. During engine starting when the disconnect clutchtorque estimate is most uncertain, much larger error bounds can beassigned. During a coasting shift which uses Target-N control, moderateerror bounds may be assigned.

This in turn improves the clutch torque estimate 408 minimizing theerror between the speed command and the actual speed. Overall thefeedback control operates to improve powertrain system robustness andtransient response.

Generally, the feedback portion of the control algorithm comprises errorcorrection to compensate for, and reduce speed discrepancies, improvingrobustness. However, the feedback portion is reactive, and there is someinherent time delay in the correction. The feedforward portion of thecontrol algorithm predicts performance and avoids some portion of theoutput speed error. As inputs change, the downstream speed output ispredicted before it arises. Feedforward control can compensate to somedegree for known motor dynamic before the errors occur, reducing systemdelays. However, feedforward control is most effective when systemresponse is highly predictable. The advance estimation of clutch torquecan carry error for several reasons. For example, the loss of a signalindicative of clutch pressure, loss of communication between controllerswithin the controller system, poor general signal conditions, and longcommunication delay all can contribute to errors in the clutch torqueestimate. When both control types are used together, the feedforwardcomponent helps to provide rapid system response, and the feedbackcomponent helps to compensate for unavoidable errors in the powertrainsystem model. The method of the present controller provides a robust wayto control hybrid powertrain output.

Speed control mode may be applied in the powertrain of the presentdisclosure to improve operation in several specific operation states oroperational modes. FIGS. 5A and 5B are a flowchart 500 of method whichillustrates an example of mode determination of speed control in thevarious powertrain operation states or operational modes.

At step 502, a controller branches based on if the powertrain is not ina speed control mode to operation 504. If the powertrain is in a speedcontrol mode, the controller will branch to operation 506A. In operation504, the controller branches to operation 506B if speed control isrequired and if speed control is not required, the controller branchesback to operation 502.

In operation 506A and 506B, the controller determines a nominal desiredreference motor speed and proceeds to operations 508A and 508Brespectively. An example of operation 506 is element 432 of FIG. 4. Inoperation 508A and 508B, the controller performs reference speedbounding. This reference speed bounding may be such that the speedcorrection decays to zero. An example of reference speed bounding iselement 434 of FIG. 4. After the reference speed bounding of operation508, the controller proceeds to operation 510 in which the controllerrecalculates the reference motor speed. The controller then measures themotor speed in operation 512, and determines the motor speed error inoperation 514.

In operation 516, the controller limits the speed error. An example ofthis limitation is element 436 of FIG. 4. Once the speed error is bound,the controller determines the speed control feedback correction torquein operation 518. An example of this is the gain block, element 428, ofFIG. 4. The controller then limits the motor feedback correction torquein operation 520. An example of applying limitation to the motorfeedback correction torque is element 442 of FIG. 4. The controller thenestimates the feedforward torque required to overcome the torqueconverter load in operation 522 and proceeds to operation 524 andestimates the feedforward disconnect clutch torque estimate. An exampleof an estimation the feedforward disconnect clutch torque estimate iselement 408 of FIG. 4. The controller then determines the final motortorque command in operation 516 and loops back to the beginning. Anexample of the determination of the final motor torque command iselement 416 of FIG. 4.

The present disclosure provides representative control strategies and/orlogic that may be implemented using one or more processing strategiessuch as event-driven, interrupt-driven, multi-tasking, multi-threading,and the like. As such, various steps or functions illustrated herein maybe performed in the sequence illustrated, in parallel, or in some casesomitted. Although not always explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending upon theparticular processing strategy being used. Similarly, the order ofprocessing is not necessarily required to achieve the features andadvantages described herein, but it is provided for ease of illustrationand description.

The control logic may be implemented primarily in software executed by amicroprocessor-based vehicle, engine, and/or powertrain controller. Ofcourse, the control logic may be implemented in software, hardware, or acombination of software and hardware in one or more controllersdepending upon the particular application. When implemented in software,the control logic may be provided in one or more computer-readablestorage devices or media having stored data representing code orinstructions executed by a computer to control the vehicle or itssubsystems. The computer-readable storage devices or media may includeone or more of a number of known physical devices which utilizeelectric, magnetic, and/or optical storage to keep executableinstructions and associated calibration information, operatingvariables, and the like. Alternatively, the processes, methods, oralgorithms can be embodied in whole or in part using suitable hardwarecomponents, such as Application Specific Integrated Circuits (ASICs),Field-Programmable Gate Arrays (FPGAs), state machines, controllers orother hardware components or devices, or a combination of hardware,software and firmware components.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

1. A vehicle powertrain comprising: an engine and electric machinecoupled by a clutch; a torque converter configured to couple theelectric machine to an output shaft; and a controller configured toalter torque output of the electric machine to reduce a differencebetween a desired rotational speed and an actual rotational speed of thetorque converter, and change thresholds that limit a maximum value ofthe difference according to powertrain operation.
 2. The vehiclepowertrain of claim 1, wherein the controller is further configured to,in response to the powertrain operation being an imminent start event ofthe engine, change the thresholds to a maximum range and command theelectric machine to increase the torque output.
 3. The vehiclepowertrain of claim 1, wherein the controller is further configured to,in response to the powertrain operation being an imminent vehicle stop,change the thresholds to a minimum range and command the electricmachine to decrease the torque output.
 4. The vehicle powertrain ofclaim 1, wherein the controller is further configured to, in response tothe powertrain operation being propulsion via the electric machine only,change the thresholds to a minimum range.
 5. The vehicle powertrain ofclaim 1, wherein the controller is further configured to, in response toa downshift of a transmission, change the thresholds to a nominal rangeand command the electric machine to output torque to drive the torqueconverter toward a predetermined rotational speed during disengagementof the clutch.
 6. The vehicle powertrain of claim 1, wherein thecontroller is further configured to limit a desired torque, derived fromthe desired rotational speed, to a range that is based on the powertrainoperation.
 7. The vehicle powertrain of claim 6, wherein the controlleris further configured to, in response to the powertrain operation beingan imminent engine start event, change the range to a maximum range. 8.The vehicle powertrain of claim 6, wherein the controller is furtherconfigured to, during vehicle travel propelled by the electric machineonly, change the range to a minimum range.
 9. A hybrid powertraincontrol method comprising: by a controller, operating an electricmachine to output a desired rotational speed that is based on a torqueestimate; altering the output responsive to a difference between thedesired rotational speed and an actual rotational speed such that thedifference is reduced; and changing thresholds that limit a range of thedifference according to an operational mode.
 10. The method of claim 9further comprising, in response to the operational mode being animminent engine start event, changing the range to a maximum.
 11. Themethod of claim 9 further comprising, in response to the operationalmode being an imminent vehicle stop, changing the range to a minimum.12. The method of claim 9 further comprising, during vehicle travelpropelled by the electric machine only, changing the range to a minimum.13. The method of claim 9 further comprising, in response to theoperational mode being transitioning from a non-motive gear to a motivegear, changing the range to a nominal range.
 14. A control system for avehicle powertrain comprising: a controller configured to operate anelectric machine responsive to a difference between a desired rotationalspeed and an actual rotational speed of a torque converter coupled withthe electric machine such that the difference is reduced, and changethresholds that limit a maximum value of the difference according to anoperational mode of the vehicle powertrain.
 15. The control system ofclaim 14, wherein the controller is further programmed to, in responseto a downshift of a transmission while vehicle speed is less than aspeed threshold, change the thresholds to a nominal range.
 16. Thecontrol system of claim 15, wherein the controller is further programmedto, in response to a shift of the transmission from a non-motive gear toa motive gear, changing the thresholds to a minimum range.
 17. Thecontrol system of claim 14, wherein the controller is further programmedto, during vehicle travel propelled by the electric machine only,changing the thresholds to a minimum range.